In forming glass fibers, a glass forming material is heated in a glass melting furnace until the glass forming material is degraded to a molten state. The molten glass is passed from the glass melting furnace and into a forehearth linking the glass melting furnace and a glass forming machine. As the molten glass flows through the forehearth, it passes downwardly through openings that are positioned in a spaced relation along the bottom of the forehearth and into a bushing. Glass streams are then attenuated through orifices located in the bushing and formed into glass filaments or fibers by methods known to those of skill in the art.
The forehearth is utilized to thermally condition the molten glass material so that it matches the physical and chemical requirements of the glass forming machine. The forehearth contains numerous openings to convey the molten glass material to the bushings. The initial or first opening is typically called the “take-off” position and the last opening in the trough of the forehearth is termed the “end position”. It is known in the art that a temperature gradient is created within the molten glass material as the molten glass material flows through the forehearth. For example, the molten glass delivery temperature at the end position is lower than the molten glass delivery temperature at other openings. It is also known that the forehearth tends to lose heat externally from the end wall, thereby causing the molten glass material at the end position to be substantially cooler and thermally less uniform compared to the molten glass located at the other openings.
The heat loss from the end position is illustrated in FIG. 1. As depicted in FIG. 1, combustion gases flow within a gas chamber 12 positioned between the roof 14 of the forehearth 10 and the molten glass forming material 16. Gas burners 15 inject flames into the gas combustion chamber 12 to elevate and maintain the temperature of the combustion gases within the gas chamber 12. The hot gases in the gas chamber 12 are utilized to maintain the temperature of the molten glass forming material 16 for optimum delivery. In FIG. 1, the molten glass material 16 flows from left to right (e.g., downstream) along a flow block 17 between two sidewalls (not shown) and abuts a glass contact wall 11. The molten glass forming material 16 exits the forehearth 10 through the end position 21 formed by an opening in the flow block 17, 17a and the bushing block 13, 13a. Heat loss occurring through the end wall 18 in the direction of arrows 19 creates a temperature gradient within the molten glass material 16 at the end position 21.
The difference in the molten glass delivery temperature across the end position 21 creates a difference in the viscosity of the molten glass 16, which may result in variations in the physical and chemical properties of the glass fibers produced as well as the end product manufactured with these glass fibers. In addition, the temperature difference may result in fibers that do not meet process specifications. Further, the reduced temperature of the molten glass material 16 at the end position 21 creates a need for increased power within the bushing to reheat the glass. This reheating, in turn, negatively alters the bushing's characteristics. Additionally, the lower temperature of the glass forming material 16 at the end position 21 causes higher shear break rates of the resulting glass fibers. Such an increase in the shear break rate results in a high level of waste and a reduction in processing efficiency.
There have been attempts in the art to reduce the temperature difference within the forehearth and overcome the above-described problems. Some examples of these attempts are described below.
U.S. Pat. No. 4,069,032 to Brax discloses an apparatus that homogenizes the temperature of the molten glass flowing through a forehearth. The inventive forehearth includes a roof that has a roof with longitudinal ridges. These ridges define a central longitudinal channel in which a current of cooling air passes therethrough to cool the central, hottest part of the stream of glass. Side channels promote a local convection of hot gases to reheat side portions of the molten glass. A plurality of longitudinally spaced electrodes are suspended from the roof over the side channels and are inserted into the molten glass to directly heat the side portions of the channel of molten glass.
U.S. Pat. No. 4,544,392 to Sheinkop discloses an apparatus for thermally conditioning a heat softenable material such as glass. The apparatus includes an auxiliary heated bushing block that has a non-circular opening to transmit the molten glass from the opening in the bottom of a forehearth to a fiber forming bushing. Electrical resistance-heater elements protrude through the ends of the bushing block into the non-circular opening into contact with the molten glass. By varying the power settings of each power supply to the heater elements, the molten glass can be selectively thermally conditioned.
U.S. Pat. No. 5,327,452 to McMinn discloses a forehearth for a glass furnace that includes a trough and a roof over the trough. Two longitudinal ridges in the roof that extend downwardly towards the surface of the molten glass form three chambers within the forehearth. The central chamber forms a conduit for the flow of cooling air over the central part of the molten glass stream. The side chambers serve as conduits for the flow of combustion gas. Separate outlets are provided for the cooling and combustion gases. Controllable dampers are provided on at least the combustion gas outlets. Balancing the internal pressures between the three chambers may ensure that there is no significant mixing of the cooling air and combustion gases. The balancing and adjusting of the pressure of adjacent heating and cooling chambers allegedly allows a fine and accurate control of the temperature of the molten glass.
Despite these previous attempts to reduce temperature differences within the forehearth, there remains a need in the art for an apparatus and method for heating the molten glass material located at the end position to provide a substantially homogeneous temperature to the molten glass forming material across the end position.